1. Field of the Invention
The present invention relates to a wired and wireless communication method, and more particularly a method and apparatus for reducing a PAPR (Peak to Average Power Ratio) in an OFDM (Orthogonal Frequency Division Multiplexing) system.
2. Description of the Related Art
In a typical parallel data processing system, an entire signal frequency band is divided into N number of frequency sub-channels, which do not overlap one another. Respective symbols are modulated through the sub-channels, and frequency division multiplexing is applied to the N sub-channels.
FIG. 1 is a view illustrating a configuration of a conventional OFDM (Orthogonal Frequency Division Multiplexing) system. Referring to FIG. 1, the conventional OFDM system modulates and demodulates parallel data using DFT (Discrete Fourier Transform).
Hereinafter, the operation of the conventional OFDM system will be described with reference to FIG. 1. At first, if a serial data stream having a predetermined size (e.g. X bits) is inputted into a data converter 11, the data converter 11 converts the serial data stream into parallel data. At this time, it is assumed that each of the parallel data outputted to output ports of the data converter 11, consists of X bits. The parallel data, which consist of X bits, respectively, are inputted into a signal mapper 12, and the signal mapper 12 modulates the parallel data on the basis of a predetermined PSK (Phase-Shift Keying) or QAM (Quadrature Amplitude Modulation) scheme, etc. and then outputs a plurality of modulated symbol streams d0, d1, . . . , dn-1. Herein, n is related to what kind of dimension is used for the modulation scheme. If BPSK is used, “n” is equal to “X”. If QPSK is used, “n” is equal to “X/2”. If 16QAM is used, “n” is equal to “X/4”. An IDFT (Inverse Discrete Fourier Transform) circuit 13 carries out IDFT for the modulated symbols d0, d1, . . . , dn-1 and then outputs parallel OFDM signals. The parallel OFDM signals are converted into a serial OFDM signals by a parallel to serial converter 14. The serial OFDM signals are inputted into a guard interval inserter 15. The guard interval inserter 15 inserts a guard interval into the serial OFDM signals and then outputs the serial OFDM signals having the guard interval to a D/A low pass filter (LPT) 16. The D/A LPT 16 converts a digital signal of the serial OFDM signals having the guard interval into an analog signal. The analog signal converted by the D/A LPT 16 is transmitted via a channel over air interface 17 via antenna. The channel refers to a data transmission channel for radio communications. Up to now, the operation of a transmitter of the OFDM communication system has been described.
The analog signal transmitted through the air interface 17 is inputted into an LPT A/D 18. The LPT A/D 18 converts the analog signal received from the channel 17 into a digital signal and then outputs the digital signal to a guard interval remover 19. The guard interval remover 19 removes the guard interval inserted into the digital signal and then outputs, to a serial to parallel converter 20, the digital signal from which the guard interval is removed. The serial to parallel converter 20 converts the digital signal from which the guard interval is removed, i.e., the serial OFDM signals, into parallel OFDM signals, and then outputs the parallel OFDM signals in a unit of d0, d1, . . . , dn-1 bits. The parallel OFDM signals are inputted into a DFT (Discrete Fourier Transform) circuit 21. The DFT circuit 21 carries out DFT for the parallel OFDM signals and then outputs Fourier Transformed symbols. The symbols are inputted into a signal demapper 22. The signal demapper 22 demodulates the symbols and then outputs the parallel data, X bits. The parallel data are inputted into a parallel to serial converter 23. The parallel to serial converter 23 converts the parallel data into one serial data stream and then outputs the serial data stream.
In an OFDM communication system using N number of carriers, it is assumed that a kth OFDM signal is represented as a modulated signal Ai,k(i=0, 1, . . . , N−1) allocated to an ith carrier in a given symbol duration T. Each modulated signal Ai,k is one of the symbols in a constellation plot based on modulation. Using the modulated signal Ai,k, an envelope having a complex value of an OFDM baseband signal is as follows.
                              s          ⁡                      (            t            )                          =                              ∑                          k              =                              -                ∞                                      ∞                    ⁢                                          ⁢                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          A                                  i                  ,                  k                                            ·                              g                ⁡                                  (                                      t                    -                    kT                                    )                                            ·                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  i                  ⁢                                                                          ⁢                                      t                    /                    T                                                                                                          [                  Equation          ⁢                                          ⁢          1                ]            In the above Equation 1, g(t) denotes a rectangular pulse having width T, and T denotes an OFDM symbol duration. To maintain the orthogonality between OFDM carriers, an ith carrier frequency fi can be represented as the following Equation 2 in terms of a center frequency fc.
                              f          i                =                              f            c                    +                      i            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            f                                              [                  Equation          ⁢                                          ⁢          2                ]            
In the above Equation 2, Δf means a bandwidth of one carrier, and is an integral multiple of an OFDM symbol rate 1/T.
Looking into several prominent characteristics of the OFDM system, when the OFDM system is compared with a single carrier system identical with the OFDM system in a transmission bandwidth and data transmission rate, a duration of one symbol to be transmitted in the OFDM system is approximately a multiple N of the duration of one symbol to be transmitted in the single carrier system, in the case where data to be transmitted is distributed on N carriers. As a result, the duration of one symbol in the OFDM system is longer than that of one symbol in the single carrier system. In addition, if a guard interval is added in a time domain, the degradation of transmission characteristics due to delay becomes less even though the number of multiple paths is increased.
Further, since data distributed on the entire transmission band is transmitted, an interference signal affects only a portion of the data in the case where the interference signal exists in a specific frequency band, and the OFDM system can efficiently improve its performance using an interleaver and an error correcting code.
Conventionally, in a multi-carrier transmission method, the peak envelope power of a multi-carrier signal increases in proportion to the number of carriers. If N signals in the OFDM system overlap in the same phase, the peak envelope power increases by a multiple of N of the average power. A PAPR is referred to as a peak to average power ratio of a multi-carrier signal. If the PAPR increases, A/D (Analog/Digital) or D/A (Digital/Analog) conversion is complicated and the efficiency of an RF (Radio Frequency) power amplifier is reduced.
Thus, research to reduce the PAPR is actively conducted and reducing the PAPR is one of problems to be necessarily addressed in order to efficiently implement the OFDM system having superior performance in RF and optical communications.
Where a symbol sequence of {A0, A1, . . . , AN-1} having complex values is transmitted through N number of carriers, an OFDM baseband signal s(t) is represented as the following Equation 3 with respect to time tε[0, T].
                              s          ⁡                      (            t            )                          =                              ∑                          i              =              0                                      N              -              1                                ⁢                                          ⁢                                    A              i                        ⁢                          ⅇ                                                j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                                ⁢                                                                                        ⁢                          f              i              t                                                          [                  Equation          ⁢                                          ⁢          3                ]            
A PAPR of the OFDM baseband signal s(t) is defined by the following Equation 4.PAPR(s)=Maximum instantaneous power of s(t)/Average power of s(t)  [Equation 4]
Referring to the above Equation 4, where Ai is an MPSK (Multiple Phase-Shift Keying) modulated symbol and the average power has a value of N, the maximum instantaneous power can have a value of N2 and the PAPR has a value of N.
Thus, if an OFDM signal is generated using a symbol sequence, it has very high maximum instantaneous power and a high PAPR. Since IDFT (Inverse Discrete Fourier Transform) and DFT are used for modulation and demodulation in the OFDM system, baseband OFDM symbols in an arbitrary symbol duration are represented as N number of sample values and hence the baseband OFDM symbols can be represented as follows.
                                          s            ⁡                          [              n              ]                                =                                    IDFT              ⁡                              (                A                )                                      =                                          1                                  N                                            ⁢                                                ∑                                      i                    =                    o                                                        N                    -                    1                                                  ⁢                                                                  ⁢                                                      A                    i                                    ⁢                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                                                        i                          ⁢                                                                                                          ⁢                          n                                                N                                                                                                                                ,                  n          =          0                ,        1        ,                                  ⁢        …        ⁢                                  ,                  N          -          1                                    [                  Equation          ⁢                                          ⁢          5                ]            
As defined in the above Equation 5, L*N-point IDFT is considered to produce the PAPR. L is an over sampling factor. If a sequence of N modulated inputs is A={A0, A1, . . . , AN-1}, a sequence A={A0, A1, . . . , ALN-1}={A0, A1, . . . , AN-1, 0, 0, . . . , 0} including (L−1)N number of 0's is considered to take the LN-point IDFT. After taking the L*N-point IDFT of the sequence A in one symbol duration using the sequence A, an nth sample is represented as follows.
            s      ⁡              [        n        ]              =                  ∑                  i          =          o                                      L            ⁢                                                  ⁢            N                    -          1                    ⁢                          ⁢                        A          i          ′                ⁢                  ⅇ                      j            ⁢                                                  ⁢            2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                                          i                ⁢                                                                  ⁢                n                            LN                                            ,      n    =    0    ,  1  ,          ⁢  …  ⁢          ,            L      ⁢                          ⁢      N        -    1  
Because the calculation of the PAPR with respect to continuous signals is complicated, the PAPR is calculated by considering only LN samples of OFDM signals associated with the sequence A. That is, the PAPR considering LN-point IDFT samples of the sequence A is defined as follows.
                                          PAPR                          L              ⁢                                                          ⁢              N                                ⁡                      (            A            )                          =                              max                                          n                =                0                            ,              1              ,                                                          ⁢              …              ⁢                                                          ,                                                          ⁢                                                L                  ⁢                                                                          ⁢                  N                                -                1                                              ⁢                                    |                              s                ⁡                                  [                  n                  ]                                            ⁢                              |                2                                                    E              ⁡                              [                                  |                                      s                    ⁡                                          [                      n                      ]                                                        ⁢                                      |                    2                                                  ]                                                                        [                  Equation          ⁢                                          ⁢          6                ]            
As defined in the above Equation 6, s[n] is a sample by the LN-point IDFT, and L denotes an oversampling factor. Further, E is an operator taking the mean of values of OFDM signal s[n] for all n. The case of L=1 is referred to as Nyquist sampling. It is well known that the PAPR can be sufficiently obtained if the oversampling factor L is 4, in order to obtain the PAPR shown in the above Equation 5 being a function of actually continuous time.
Several methods for reducing the PAPR in the OFDM communication system have been suggested. In a simplest method for reducing the PAPR, signal clipping is considered to limit a maximum size of a signal to a predetermined size or less.
The conventional clipping is the simplest method for reducing the PAPR, but has several problems. At first, the clipping causes amplitude of an OFDM signal to be distorted, and hence self-interference is generated to increase a BER (Bit Error Rate). Further, because the distortion of the OFDM signal is non-linear, it causes out-of-band frequency characteristics to be degraded.
On the other hand, in another method for reducing the PAPR in the OFDM system, a Golay sequence becomes an important factor in reducing the PAPR in the OFDM system. Where only the Golay sequence is used, there is an advantage in that a value of the PAPR is limited to 2 (3 dB). However, there is a disadvantage in that a code rate is rapidly reduced as the number of carriers increases.
A conventional error-correcting code technique can be used to reduce the PAPR in the OFDM system. In the conventional technique, only a codeword having peak envelope power of a small value is selected such that an OFDM signal can be generated to reduce the entire PAPR. However, there is a problem in that a code rate is greatly reduced as the number of carrier signals increases.
There is SLM (Selective Mapping) as another conventional technique. A basic concept of the SLM is to generate a plurality of OFDM signals indicating the same information. That is, the SLM generates U number of sequences indicating the same information and transmits a sequence having a smallest PAPR among the U sequences.
FIG. 2 is a view illustrating a structure of a conventional OFDM system based on the SLM. If an input sequence is denoted A, U number of independent sequences with the above-described input sequence are generated by multiplying a sequence of P(u) by the input sequence A, where u=1, 2, . . . , U. As a result, U number of sequences a(u) for u=1, 2, . . . , U in a time domain are produced by the IDFT of each sequence A(u).
Hereinafter, the operation of a transmitter of the conventional SLM-based OFDM system will be described with reference to FIG. 2. At first, a serial data stream A having n number of digital value (1 or −1) from a data source 31 is inputted into a serial to parallel converter 32. The serial to parallel converter 32 converts the serial data stream into parallel data (n)and then outputs the parallel data. Each of the parallel data is inputted into a corresponding one of multipliers 33-1, 33-2, . . . , 33-U. Each of the multipliers 33-1, 33-2, . . . , 33-U multiply the parallel data (n)by an external input sequence element by element, respectively. The first multiplier 33-1 multiplies, element by element, the parallel data (n) by a first sequence P(1) having “n” number of elements and then outputs a sequence A(1) having “n” number of elements. The second multiplier 33-2 multiplies, element by element, the parallel data (n) by a second sequence P(2) having “n” number of elements and then outputs a sequence A(2) having “n” number of elements. The Uth multiplier 33-U multiplies, element by element, the parallel data (n) by a Uth sequence P(U) having “n” number of elements and then outputs a sequence A(U) having “n” number of elements. The operations of the third to (U−1)th multipliers are similar with those of the above-described multipliers. The sequences A(1), A(2), . . . , A(U) outputted from the multipliers 33-1, 33-2, . . . , 33-U are inputted into a corresponding one of U IDFT circuits 34-1, 34-2, . . . , 34-U, respectively. The IDFT circuits 34-1, 34-2, . . . , 34-U carry out IDFT for sequences A(1), A(2), . . . , A(U) and then outputs IDFT signals, i.e., sequences a(1), a(2), . . . , a(U) in a time domain. The sequences a(1), a(2), . . . , a(U) in the time domain are inputted into a selector 35. The selector calculates PAPRs for the sequences a(1), a(2), . . . , a(U) and then selects a PAPR having the smallest value among the calculated PAPRs. If the PAPR having the smallest value is selected, an IDFT signal stream having the selected PAPR is outputted as a final transmission signal a(u). Simultaneously, side information is transmitted along with the IDFT signal stream having the selected PAPR.
The U number of sequences a(u) for u=1, 2, . . . , U in the time domain produced by the IDFT of each sequence A(u) is given by Eq. 7:a(u)=IDFT{A(u)}, u=1,2 , . . . ,U  [Equation 7]
All sequences a(1) to a(u) including the same information A, and a signal to be actually transmitted among the sequences a(1) to a(u) has the smallest PAPR value.
Theoretically, as the number of independent sequences, i.e., a value of U, increases, the characteristic of the PAPR becomes better. However, if the value of U increases, the complexity of the system increases.
Also, there is another significant disadvantage. Side information must be transmitted from a transmitter to a receiver because the receiver must identify which sequence is actually used at the transmitter among U number of sequence to recover original information. Moreover, if the side information have an error during transmission, a burst error can be caused, thereby greatly degrading the system's performance. Thus, the side information should be highly protected during transmission.
There is a PTS (Partial Transmit Sequence) method as another conventional technique. The conventional PTS method divides an input sequence into M number of independent and partial blocks and then shifts a phase of each partial block, thereby generating a plurality of sequences and reducing the PAPR. Side information associated with the phase shift should also be transmitted such that a receiver can recover original information.